Major Modification: This term is used to define modifications
of major stationary sources of emissions with respect to Prevention of
Significant Deterioration and New Source Review under the Clean Air Act.

Major Stationary Sources: Term used to determine the applicability
of Prevention of Significant Deterioration and new source regulations.
In a nonattainment area, any stationary pollutant source with potential
to emit more than 100 tons per year is considered a major stationary source.
In PSD areas the cutoff level may be either 100 or 250 tons, depending
upon the source.

Majors: Larger publicly owned treatment works (POTWs) with flows
equal to at least one million gallons per day (mgd) or servicing a population
equivalent to 10,000 persons; certain other POTWs having significant water
quality impacts. (See: minors.)

Man-Made (Anthropogenic) Beta Particle and Photon Emitters: All
radionuclides emitting beta particles and/or photons listed in Maximum
Permissible Body Burdens and Maximum Permissible Concentrations of Radonuclides
in Air and Water for Occupational Exposure.

Management Plan: Under the Asbestos Hazard Emergency Response
Act (AHERA), a document that each Local Education Agency is required to
prepare, describing all activities planned and undertaken by a school
to comply with AHERA regulations, including building inspections to identify
asbestos-containing materials, response actions, and operations and maintenance
programs to minimize the risk of exposure.

Managerial Controls: Methods of nonpoint source pollution control
based on decisions about managing agricultural wastes or application times
or rates for agrochemicals.

Mandatory Recycling: Programs which by law require consumers to
separate trash so that some or all recyclable materials are recovered
for recycling rather than going to landfills.

Manifest: A one-page form used by haulers transporting waste that
lists EPA identification numbers, type and quantity of waste, the generator
it originated from, the transporter that shipped it, and the storage or
disposal facility to which it is being shipped. It includes copies for
all participants in the shipping process.

Manifest System: Tracking of hazardous waste from "cradle-to-grave"
(generation through disposal) with accompanying documents known as manifests.(See:
cradle to grave.)

Material Type: Classification of suspect material by its specific
use or application; e.g., pipe insulation, fireproofing, and floor tile.

Materials Recovery Facility (MRF): A facility that processes residentially
collected mixed recyclables into new products available for market.

Maximally (or Most) Exposed Individual: The person with the highest
exposure in a given population.

Maximum Acceptable Toxic Concentration: For a given ecological
effects test, the range (or geometric mean) between the No Observable
Adverse Effect Level and the Lowest Observable Adverse Effects Level.

Maximum Available Control Technology (MACT): The emission standard
for sources of air pollution requiring the maximum reduction of hazardous
emissions, taking cost and feasibility into account. Under the Clean Air
Act Amendments of 1990, the MACT must not be less than the average emission
level achieved by controls on the best performing 12 percent of existing
sources, by category of industrial and utility sources.

Maximum Contaminant Level: The maximum permissible level of a
contaminant in water delivered to any user of a public system. MCLs are
enforceable standards.

Maximum Contaminant Level Goal (MCLG): Under the Safe Drinking
Water Act, a non-enforceable concentration of a drinking water contaminant,
set at the level at which no known or anticipated adverse effects on human
health occur and which allows an adequate safety margin. The MCLG is usually
the starting point for determining the regulated Maximum Contaminant Level.
(See: maximum contaminant level.)

Maximum Exposure Range: Estimate of exposure or dose level received
by an individual in a defined population that is greater than the 98th
percentile dose for all individuals in that population, but less than
the exposure level received by the person receiving the highest exposure
level.

Maximum Residue Level: Comparable to a U.S. tolerance level, the
Maximum Residue Level the enforceable limit on food pesticide levels in
some countries. Levels are set by the Codex Alimentarius Commission, a
United Nations agency managed and funded jointly by the World Health Organization
and the Food and Agriculture Organization.

Maximum Tolerated Dose: The maximum dose that an animal species
can tolerate for a major portion of its lifetime without significant impairment
or toxic effect other than carcinogenicity.

Measure of Effect/ Measurement Endpoint: A measurable characteristic
of ecological entity that can be related to an assessment endpoint; e.g.
a laboratory test for eight species meeting certain requirements may serve
as a measure of effect for an assessment endpoint, such as survival of
fish, aquatic, invertebrate or algal species under acute exposure.

Measure of Exposure: A measurable characteristic of a stressor
(such as the specific amount of mercury in a body of water) used to help
quantify the exposure of an ecological entity or individual organism.

Mechanical Aeration: Use of mechanical energy to inject air into
water to cause a waste stream to absorb oxygen.

Mechanical Separation: Using mechanical means to separate waste
into various components.

Mechanical Turbulence: Random irregularities of fluid motion in
air caused by buildings or other nonthermal, processes.

Media: Specific environments--air, water, soil--which are the
subject of regulatory concern and activities.

Medical Surveillance: A periodic comprehensive review of a worker's
health status; acceptable elements of such surveillance program are listed
in the Occupational Safety and Health Administration standards for asbestos.

Medical Waste: Any solid waste generated in the diagnosis, treatment,
or immunization of human beings or animals, in research pertaining thereto,
or in the production or testing of biologicals, excluding hazardous waste
identified or listed under 40 CFR Part 261 or any household waste as defined
in 40 CFR Sub-section 261.4 (b)(1).

Medium-size Water System: A water system that serves 3,300 to
50,000 customers.

Meniscus: The curved top of a column of liquid in a small tube.

Mercury (Hg): Heavy metal that can accumulate in the environment
and is highly toxic if breathed or swallowed. (See:heavy
metals.)

Mesotrophic: Reservoirs and lakes which contain moderate quantities
of nutrients and are moderately productive in terms of aquatic animal
and plant life.

Metabolites: Any substances produced by biological processes,
such as those from pesticides.

Metalimnion: The middle layer of a thermally stratified lake or
reservoir. In this layer there is a rapid decrease in temperature with
depth. Also called thermocline.

Methane: A colorless, nonpoisonous, flammable gas created by anaerobic
decomposition of organic compounds. A major component of natural gas used
in the home.

Methanol: An alcohol that can be used as an alternative fuel or
as a gasoline additive. It is less volatile than gasoline; when blended
with gasoline it lowers the carbon monoxide emissions but increases hydrocarbon
emissions. Used as pure fuel, its emissions are less ozone-forming than
those from gasoline. Poisonous to humans and animals if ingested.

Method 18: An EPA test method which uses gas chromatographic techniques
to measure the concentration of volatile organic compounds in a gas stream.

Method 24: An EPA reference method to determine density, water
content and total volatile content (water and VOC) of coatings.

Method 25: An EPA reference method to determine the VOC concentration
in a gas stream.

Method Detection Limit (MDL): See limit of detection.

Methoxychlor: Pesticide that causes adverse health effects in
domestic water supplies and is toxic to freshwater and marine aquatic
life.

Methyl Orange Alkalinity: A measure of the total alkalinity in
a water sample in which the color of methyl orange reflects the change
in level.

Microbial Growth: The amplification or multiplication of microorganisms
such as bacteria, algae, diatoms, plankton, and fungi.

Microbial Pesticide: A microorganism that is used to kill a pest,
but is of minimum toxicity to humans.

Microclimate: 1. Localized climate conditions within an urban
area or neighborhood. 2. The climate around a tree or shrub or a stand
of trees.

Microenvironmental Method: A method for sequentially assessing
exposure for a series of microenvironments that can be approximated by
constant concentrations of a stressor.

Microenvironments: Well-defined surroundings such as the home,
office, or kitchen that can be treated as uniform in terms of stressor
concentration.

Million-Gallons Per Day (MGD): A measure of water flow.

Minimization: A comprehensive program to minimize or eliminate
wastes, usually applied to wastes at their point of origin. (See: waste
minimization.)

Mining of an Aquifer: Withdrawal over a period of time of ground
water that exceeds the rate of recharge of the aquifer.

Mining Waste: Residues resulting from the extraction of raw materials
from the earth.

Minor Source: New emissions sources or modifications to existing
emissions sources that do not exceed NAAQS emission levels.

Minors: Publicly owned treatment works with flows less than 1
million gallons per day. (See: majors.)

Miscellaneous ACM: Interior asbestos-containing building material
or structural components, members or fixtures, such as floor and ceiling
tiles; does not include surfacing materials or thermal system insulation.

Miscellaneous Materials: Interior building materials on structural
components, such as floor or ceiling tiles.

Miscible Liquids: Two or more liquids that can be mixed and will
remain mixed under normal conditions.

Missed Detection: The situation that occurs when a test indicates
that a tank is "tight" when in fact it is leaking.

Mist: Liquid particles measuring 40 to 500 micrometers (pm), are
formed by condensation of vapor. By comparison, fog particles are smaller
than 40 micrometers (pm).

Mitigation: Measures taken to reduce adverse impacts on the environment.

Mixed Funding: Settlements in which potentially responsible parties
and EPA share the cost of a response action.

Mixed Liquor: A mixture of activated sludge and water containing
organic matter undergoing activated sludge treatment in an aeration tank.

Mixed Metals: Recovered metals not sorted into categories such
as aluminum, tin, or steel cans or ferrous or non-ferrous metals.

Mixed Municipal Waste: Solid waste that has not been sorted into
specific categories (such as plastic, glass, yard trimmings, etc.)

Mixed Paper: Recovered paper not sorted into categories such as
old magazines, old newspapers, old corrugated boxes, etc.

Mixed Plastic: Recovered plastic unsorted by category.

Mobile Incinerator Systems: Hazardous waste incinerators that
can be transported from one site to another.

Mobile Source: Any non-stationary source of air pollution such
as cars, trucks, motorcycles, buses, airplanes, and locomotives.

Model Plant: A hypothetical plant design used for developing economic,
environmental, and energy impact analyses as support for regulations or
regulatory guidelines; first step in exploring the economic impact of
a potential NSPS.

Modified Bin Method: Way of calculating the required heating or
cooling for a building based on determining how much energy the system
would use if outdoor temperatures were within a certain temperature interval
and then multiplying the energy use by the time the temperature interval
typically occurs.

Modified Source: The enlargement of a major stationary pollutant
sources is often referred to as modification, implying that more emissions
will occur.

Moisture Content: 1.The amount of water lost from soil upon drying
to a constant weight, expressed as the weight per unit of dry soil or
as the volume of water per unit bulk volume of the soil. For a fully saturated
medium, moisture content indicates the porosity. 2. Water equivalent of
snow on the ground; an indicator of snowmelt flood potential.

Molecule: The smallest division of a compound that still retains
or exhibits all the properties of the substance.

Monitoring: Periodic or continuous surveillance or testing to
determine the level of compliance with statutory requirements and/or pollutant
levels in various media or in humans, plants, and animals.

Monitoring Well: 1. A well used to obtain water quality samples
or measure groundwater levels. 2. A well drilled at a hazardous waste
management facility or Superfund site to collect ground-water samples
for the purpose of physical, chemical, or biological analysis to determine
the amounts, types, and distribution of contaminants in the groundwater
beneath the site.

Monoclonal Antibodies (Also called MABs and MCAs): 1. Man-made
(anthropogenic) clones of a molecule, produced in quantity for medical
or research purposes. 2. Molecules of living organisms that selectively
find and attach to other molecules to which their structure conforms exactly.
This could also apply to equivalent activity by chemical molecules.

Monomictic: Lakes and reservoirs which are relatively deep, do
not freeze over during winter, and undergo a single stratification and
mixing cycle during the year (usually in the fall).

Montreal Protocol: Treaty, signed in 1987, governs stratospheric
ozone protection and research, and the production and use of ozone-depleting
substances. It provides for the end of production of ozone-depleting substances
such as CFCS. Under the Protocol, various research groups continue to
assess the ozone layer. The Multilateral Fund provides resources to developing
nations to promote the transition to ozone-safe technologies.

Moratorium: During the negotiation process, a period of 60 to
90 days during which EPA and potentially responsible parties may reach
settlement but no site response activities can be conducted.

Morbidity: Rate of disease incidence.

Mortality: Death rate.

Most Probable Number: An estimate of microbial density per unit
volume of water sample, based on probability theory.

Muck Soils: Earth made from decaying plant materials.

Mudballs: Round material that forms in filters and gradually increases
in size when not removed by backwashing.

Multi-Media Approach: Joint approach to several environmental
media, such as air, water, and land.

Multiple Chemical Sensitivity: A diagnostic label for people who
suffer multi-system illnesses as a result of contact with, or proximity
to, a variety of airborne agents and other substances.

Multiple Use: Use of land for more than one purpose; e.g., grazing
of livestock, watershed and wildlife protection, recreation, and timber
production. Also applies to use of bodies of water for recreational purposes,
fishing, and water supply.

Multistage Remote Sensing: A strategy for landscape characterization
that involves gathering and analyzing information at several geographic
scales, ranging from generalized levels of detail at the national level
through high levels of detail at the local scale.

Municipal Discharge: Discharge of effluent from waste water treatment
plants which receive waste water from households, commercial establishments,
and industries in the coastal drainage basin. Combined sewer/separate
storm overflows are included in this category.

Mutagen/Mutagenicity: An agent that causes a permanent genetic
change in a cell other than that which occurs during normal growth. Mutagenicity
is the capacity of a chemical or physical agent to cause such permanent
changes.